1. Field of the Invention
This invention relates to a new and improved pre-stressed tie rod and a method of manufacturing the same. More particularly, this invention is directed toward use of an improved pre-stressed tie rod to increase the capacity of plastic molding machines to mold larger parts. Still more particularly, the present invention can also be applied to other types of devices such as presses.
2. Prior Art
Injection molding machines are categorized by the amount of force they are able to exert on a work piece or molding dies. Generally speaking Injection molding machines fall into two types. The first type, the direct force injection molding machine, has been around for many years and uses a large hydraulic cylinder to directly exert a large force on the mold. The second type, the restrained force injection molding machine, is a relatively new concept which uses the strength of tie rods to restrain the molds against the forces created by the molten plastic or other material forced into the die.
The direct force injection molding machine has a static platen, a dynamic platen, a base, a mold or die with two halves and a source of force such as a large hydraulic cylinder. The hydraulic cylinder and static platen are anchored on the base. The hydraulic cylinder moves the dynamic platen in relationship to the static platen. When in use the two halves of the die are contained between the static platen and the dynamic platen. The hydraulic cylinder exerts a force on the dynamic platen which forces the two halves of the die together. The force exerted by the hydraulic cylinder must be greater than the force exerted by the hot molten plastic injected into the die. If it is not the plastic will seep between the two halves of the mold and cause the parts being formed to flash. If the amount of flashing is too great, the parts must be discarded.
The greatest short coming of the direct force injection molding machine is the high energy costs. When the piece is being formed the hydraulic cylinder must maintain the force on the platens and mold the entire time. This requires a large hydraulic pump powering a large hydraulic cylinder to achieve high clamping loads. This translates into higher energy costs to run the molding operation.
The second type of injection molding machine, the restrained force injection molding machine, relies on the strength of the tie rods to hold the two platens and two mold halves together during the molding process. It does not require large hydraulic cylinder to apply force throughout the molding cycle. It uses much smaller hydraulic cylinders to move and lock into place the dynamic platen and the attached half of the mold, the energy consumed by those hydraulic cylinders is much less than the energy consumed using a similar sized direct force injection molding machine.
U.S. Pat. No. 6,241,508, entitled “Multiple Mold Workstation with single Injection Feeder and Hydraulic Pumping Station”, issued to John Michael et al., which is incorporated herein by reference, discloses a restrained force injection molding machine. The primary limiting factors in the ability of a restrained force injection molding machine is the capacity of the tie rods. The capacity of the tie rods is determined by the number of tie rods, the cross-sectional area of each tie rod, and the Young's modulus, or otherwise known as the modulus elasticity of the material from which the tie rods are constructed.
One way to increase the capacity of tie rods in other types of devices is to have a pre-stressed tie rod. However, heretofore use of pre-stressed tie rods on injection molding machines is not known. U.S. Pat. No. 4,240,342, entitled Frame Structure for a Press Assembly, issued to Philip T. Delmer on Dec. 23, 1980. The Delmer patent discloses a press assembly having an improved frame, including a crown, bed, cylinder and ram assembly, tied together by tie rods and compression members in which a platen assembly secured to the ram assembly guides directly on the inward facing surface portion of the tie rods.
U.S. Pat. No. 6,250,216, entitled Press Deflection Controller and Method of Controlling Press Deflection, issued to John B. Bornhorst on Jun. 26, 2001. The Bornhorst patent discloses a mechanical press having a press deflection controller. The press includes press members which have work surfaces, such as a slide in a bed. The press deflection controller includes a tie rod which is encased in a tube. The tie rod is connected to the press member and is maintained in tension while the tube is maintained in compression. Adjusting the tension in the tie rod during press operation works to adjust the deflection in the press member.
The Bornhorst patent differs from the Delmer patent in that the tie rods in the Delmer patent run parallel with the direction of the force being applied whereas the pre-stressed tie rod in Bornhorst runs perpendicular to the direction of the force being applied.
When the press is in use, the tie rods are in tension. The tension force in the tie rods is equal to the compression force being exerted on the work piece. The capacity of the press for exerting force on the work piece is limited by the tensile strength of the tie rods. The tensile capacity of the tie rods can be increased by pre-loading the tie rods with compression. When the tie rods have been pre-loaded with a compression force, the total tensile capacity of the tie rod is then equal to the original tensile strength of the tie rod plus the pre-loaded compressive force in the tie rod.
The pre-stressed tie rods disclosed in Delmer, Bornhorst and other prior art typically include a center portion which is threaded on both ends. The center portion is then extended through the center of a tubular member. A nut or other threaded fixture is then engaged on the threads on either end of the center piece. As the nuts travel along the threads of the center piece, they travel toward one another, capturing the tubular member between them. Once the nuts engage the tubular member, they exert a force on the tubular member. This creates a tension in the center piece which is equal to the compression exerted on the tubular members. One of the shortcomings of the prior art is that they rely upon the movement of the nut on the threaded centerpiece to tension the center portion and compress the outer tubular member. The force exerted by this arrangement can be difficult to gauge and balance as well as adjust. This oftentimes leads to less than optimal use of the device.
Because of the innate inaccuracy and difficulty of use of prior art pre-stressed tie rods, heretofore restrained force injection molding machines use single piece tie rods which are not pre-stressed. FIG. 1 shows a restrained force injection molding machine 20. FIG. 2 shows a partial top view of the restrained force injection molding machine shown in FIG. 1. The shortcomings of the prior art tie rods did not allow for the use of pre-stressed tie rods on an injection molding machine.
As can best be seen in FIG. 2, there is a stationary platen 26. The stationary platen 26 serves as a work surface for the injection molding machine 20. The base of the mold 28 is attached to the stationary platen 26. Four tie rods 30 extend perpendicular from the stationary platen 26. The dynamic platen 32 moves in relationship to the stationary platen 26 along the tie rods 30. The second half of the mold 34 is attached to the interior surface of the dynamic platen 32.
In operation, the dynamic platen 32 is moved toward the stationary platen 26 by a hydraulic cylinder 36 until the base and top of the mold 28 and 34 are in contact with one another. Each tie rod 30 has a collar 38 which is fixedly attached to the tie rod 30. The base of the mold 28 and the second half of the mold 34 are forced together by a locking mechanism 40. The locking mechanism 40 is comprised of a hydraulic cylinder 42, with a wedge 44 located on either end of the hydraulic cylinder 42. The locking mechanism 40 creates a force holding the base of the mold 28 and second half of the mold 34 together by the hydraulic cylinder 42 forcing the wedge 44 between the outside surface of the dynamic platen 32 and the collar 38. This places the base and second half of the mold 28 and 34 into compression. At the same time, it puts the tie rods 30 into tension. The tension in the tie rods 30 is equal to the compression force exerted on the base of the mold 28 and second half of the mold 34.
A hot molten plastic is then pumped into the mold at very high pressure. The tie rods 30 deflect in proportion to the force exerted by the hot molten plastic. The force created by the pressure of the molten plastic will cause the tie rod 30 to deflect, causing a gap between the base of the mold 28 and the second half of the mold 34. If the gap is large enough the molten plastic will flash between the base of the mold 28 and the second half of the mold 34. If the flashing is large enough it can take the part being formed out of acceptable tolerances, in which case the part must be discarded.
The deflection of the tie rods 30 can be calculated by the following equation:
  δ  =      PL    AE  wherein δ is equal to the length of the deflection, P is equal to the force applied, L is equal to the length of the tie rod, A is equal to the cross-sectional area of the tie rod, and E is equal to the modulus of elasticity of the material from which the tie rods are made.
As can be seen by the equation, one of the limiting factors in determining the deflection is the length of the tie rod. For this reason, restrained force injection molding machine as shown in FIG. 1 typically use a rather short length tie rod 30. This in turn limits the height of the piece which can be formed. For this reason, restrained force injection molding machine such as the one shown in FIG. 1 have been limited to applications such as molding pallets or other short or flat objects. This limitation has prevented them from being used to mold taller objects such as traffic barrels.